The error, due to a problem called “sensor drift,” began in early January and caused a slowly growing underestimation of sea ice extent until mid-February. That’s when “puzzled readers” alerted the NSIDC about data showing ice-covered areas as stretches of open ocean, the Boulder, Colorado-based group said on its Web site.

“Sensor drift, although infrequent, does occasionally occur and it is one of the things that we account for during quality- control measures prior to archiving the data,” the center said. “Although we believe that data prior to early January are reliable, we will conduct a full quality check.’’

The extent of Arctic sea ice is seen as a key measure of how rising temperatures are affecting the Earth. The cap retreated in 2007 to its lowest extent ever and last year posted its second- lowest annual minimum at the end of the yearly melt season. The recent error doesn’t change findings that Arctic ice is retreating, the NSIDC said.

The center said real-time data on sea ice is always less reliable than archived numbers because full checks haven’t yet been carried out. Historical data is checked across other sources, it said.

The NSIDC uses Department of Defense satellites to obtain its Arctic sea ice data rather than more accurate National Aeronautics and Space Administration equipment. That’s because the defense satellites have a longer period of historical data, enabling scientists to draw conclusions about long-term ice melt, the center said.

“There is a balance between being as accurate as possible at any given moment and being as consistent as possible through long time-periods,” NSIDC said. “Our main scientific focus is on the long-term changes in Arctic sea ice.”